Various industrial processes use, produce, or handle gas streams that contain solid particles. For example, chemical plants, pipeline stations, gasoline plants, power generation plants, refineries, and town border stations for distribution systems have gas streams that contain solid particles such as dust, dirt and scale. For several reasons, it may be desirable to remove the solid particles from the gas stream. At power plants, for example, dust collection equipment removes fly-ash and other pollutants from flue gases in order to control air pollution. At plants that have large engines, engine intake air is filtered to reduce the amount of particulate matter entering the engine, thus, reducing equipment maintenance and extending the overall life of the equipment. In metallurgical operations that generate siliceous and metallic dusts, dust collection equipment is also used to reduce health and safety hazards. As a final example, in industries, such as the pharmaceutical industry, product quality requirements may demand the removal of solids from a gas stream. These various industrial processes attempt to remove the solid particles from gas streams in a cost efficient manner. Typically, gas filters are used to remove such undesirable solids from the gas streams. One common type of gas filter includes a filter vessel with a multi-cylindrical filter cartridge assembly for capturing the undesirable solids.
In gas filter operation, the dirty gas enters the filter vessel through an inlet nozzle. The gas expands as it travels from a relatively smaller inlet nozzle into a larger filter vessel. Once in the filter vessel, the gas flows between the filter cartridges and through a porous filter material of the filter cartridge. Typically, the outer portion of the porous filter material is in a cylindrical configuration. The porous filter material includes materials such as woven cloth, felt or other porous membrane materials through which the gas passes but through which the solids will not pass. In other words, the solids are deposited on the filtration material. The filtered gas then flows through a hollow section of the cylindrical filter cartridges and exits the filter vessel through an outlet nozzle as clean gas. In some gas filters, the gas flows from the cylindrical filter cartridges through a standpipe before exiting the filter vessel.
The process of cleaning a gas stream by passing it through a gas filter affects the flow properties of the gas stream. Specifically, the gas filtration process causes the pressure of the gas to drop as the gas flows within the filter vessel. This pressure drop is typically a function of velocity. As velocity of a stream of gas increases its pressure losses also increase, and the overall pressure of the system drops. And, conversely, as the velocity of a stream of gas decreases, its pressure losses also decrease resulting in a smaller overall system pressure drop. Therefore, as the gas moves from the outer portions of the vessel and travels between the cylindrical filter cartridges, the gas contracts, its velocity increases and its pressure drops (contraction pressure drop). The smaller the open space between the cylindrical filter cartridges within the filter vessel, the higher the pressure drop will be because the velocity of the gas increases as the gas stream travels through the restricted space. The converse is also true; that is, the larger the open space between the cylindrical filter cartridges within the filter vessel, the lower the velocity and corresponding pressure drop will be. Thus, the pressure drop of the gas as it flows through the filter vessel may be minimized, for example, by increasing the proportion of open space in the filter vessel in relation to the space occupied by the filter cartridges, commensurate with the efficient operation of the underlying system.
As the gas passes through the filter media, a further pressure drop occurs (inertial pressure drop). If the surface area of the filter media is small, the gas velocity will be high (due to the restricted passage) and, thus, the pressure drop will also be high. Conversely, if the surface area of the filter media is larger, the gas velocity and corresponding pressure drop will be lower. Thus, to minimize the inertial pressure drop, it is advantageous to increase the surface area of the filter media to a particular point commensurate with the efficient operation of the underlying system.
After filtering through the filter cartridges, a further pressure drop occurs as the gas enters into more open space in the filter vessel. As the gas moves through the standpipes from the filter cartridges into more open space its pressure decrease and finally as the gas expands its velocity and corresponding pressure drop additionally decrease (expansion pressure drop). Thus, the pressure drop of the gas as it flows through the filter vessel may be minimized by increasing the standpipe opening area in relation to the open space in the filter vessel, commensurate with the efficient operation of the underlying system. It should be noted that typically the diameter of the standpipe increases with an increase in the diameter of a filter cartridge.
To summarize, the extent of the pressure drop of a gas as it travels through a filter vessel is a function of, among other parameters, the amount of open space in the filter vessel in relation to the space occupied by filter cartridges, the amount of open space in the filter vessel in relation to the standpipe opening area through which the gas travels and the total filter medium surface area of the filter cartridges. The total pressure drop experienced in a filter vessel may, therefore, be controlled by varying any one or a combination of the diameters of filter cartridges, the amount of filter cartridges, or changing the filter vessel diameter.
Historically, when designing gas filters for a particular process, the end user usually specifies the maximum allowable pressure drop that the filter should cause when the filter is installed in the process. Additionally, the end user may specify the minimum life expectancy of the filter cartridges during operation of the filter in the process. To meet these criteria, the filter provider historically used the smallest sized filter vessel densely packed with cylindrical filter cartridges to an extent that the filter provider believes might meet the end user's maximum allowable pressure drop and filter cartridge life requirements. If these requirements are not met initially, the filter provider would usually vary the original configuration of filter cartridges until the end user's requirements were achieved. For example, if the pressure drop is higher than what the end user requires, the filter provider may increase the diameter of the cylindrical filter cartridges, which reduces the overall number of filter cartridges within the filter vessel in order to achieve a lower pressure drop. Another way of increasing open space is to increase the filter vessel size. Increasing the filter vessel size, however, was not a favored procedure because the filter vessel is one of the more expensive components of a gas filter.
Over time, filter designers started focusing more on utilizing the smallest filter vessel size possible to achieve a given pressure drop because, as noted above, the filter vessel is one of the more expensive components of the gas filter. As a first step, the trial and error method noted above was replaced with more sophisticated methods of determining the optimal filter cartridge diameter to stay within a desired pressure drop for a given vessel size. These methods include calculations and/or simulations of the filter in service. For example, FIG. 10 shows a graph produced from data gathered in a filter optimization process. For a given filter vessel diameter “D”, filter cartridges of diameter “d” are loaded into the filter vessel and the pressure drop “ΔP” caused by the gas filter is measured. Alternatively, the pressure drop may be calculated based on the parameters of the gas filter configuration being examined. Filter cartridge diameter “d” is varied several times and a plot of pressure drop against filter diameter “d” produces curve “A.” Point “B” on curve “A” is where the lowest pressure drop for filter vessel diameter “D” may be achieved. As such, filter diameter “d0” is the optimal filter cartridge diameter that will provide the lowest pressure drop. The optimization process may be continued by reducing vessel diameter “D” and repeating the calculations or simulations to determine the optimal filter vessel diameter. Consequently, for an optimized pressure drop, the filter designer is able to specify with considerable accuracy what the optimal vessel diameter and optimal cylindrical filter cartridge diameter should be. There is a continuing need, however, to further optimize the performance of gas filters.